The present invention relates to a copier, laser printer or similar image forming apparatus of the type having an intermediate transfer body in the form of a belt and, more particularly, to an image forming apparatus operable in a page-sequential paper discharge mode which discharges recording media, or paper sheets, in order of page.
An image forming apparatus of the type described is disclosed in Japanese Patent Laid-Open Publication Nos. 209470/1989 to 209472/1989. With this type of apparatus such as a copier, it is a common practice to execute a two-sided copy mode by forming a non-mirror toner image (first image) representative of a first document on a photoconductive element, then transferring the toner image to an intermediate transfer belt, and then forming a mirror toner image (second image) representative of the second document on the photoconductive element. The first and second images so formed on the transfer belt and drum, respectively, are simultaneously transferred to opposite sides of a recording medium or paper sheet. Such a conventional apparatus has various problems left unsolved, as enumerated below.
(1) When black-and-white documents and color documents are stacked together, the image surface and the lead-and-trail orientation of a discharged paper sheet differs depending on the document. PA1 (2) Although a paper reversing means may be used to turn over paper sheets so that their image surfaces may coincide, the paper sheets are sill different in lead-and-trail orientation from one another and, therefore, cannot be arranged in order of page. PA1 (3) Should a toner image representative of a black-and-white document be transferred to the transfer belt in the same manner as a toner image representative of a color document, the productivity (copying rate) would be critically lowered although paper sheets could be discharged in order of page despite the mixture of two different kinds of documents. PA1 (4) While a copier with an automatic document feeder which allows paper sheets to be driven out in order of page is extensively used, such a document feeder is not applicable to a copier of the type using a transfer belt. PA1 (5) When a two-sided document carrying a black-and-white and a color image on opposite sides thereof is used, a paper sheet carrying an image on the first side thereof is stacked in a particular position, depending on the kind of image printed on the second side of the document. Hence, it is necessary to confirm whether the images printed on both sides of documents are black-and-white or color. PA1 (6) The conventional apparatus does not need stacking means for temporarily stacking paper sheets each carrying an image on one side thereof in order to produce a two-sided copy or a composite copy and, therefore, paper transport paths extending to and from the stacking means. The construction is, therefore, simple and miniature while achieving acceptable reliability as to paper jams and other occurrences. However, a plurality of copies cannot be reproduced unless the documents are replaced, resulting in lower productivity compared to the copier with refeedable stacking means. PA1 (7) Paper sheets having undergone copying operation may be rearranged in order of page by hand. However, when a finisher having a punching function or a stapling function, for example, is associated with the copier, such manual rearrangement of paper sheets is a vital problem.
Further, the conventional apparatus taught in previously mentioned Laid-Open Publications each has optics which illuminates a document from both of the leading and trailing edges thereof and focuses the resultant reflection onto a photoconductive element. With this kind of optics, it is impracticable to enhance reproducibility unless a slit having an extremely narrow width is used.